Modern integrated circuits are formed on semiconductor chips. To increase manufacturing throughput and to lower manufacturing costs, the integrated circuits are manufactured in semiconductor wafers, each containing many identical semiconductor chips. After the integrated circuits are manufactured, semiconductor chips are sawed from the wafers and packaged before they can be used.
In typical packaging processes, semiconductor chips (also referred to as dies in the art) are first attached to package substrates. This includes physically securing the semiconductor chips on the package substrates, and connecting bonding pads on the semiconductor chips to bonding pads on the package substrates. Underfill, which typically comprises epoxy, is used to further secure the bonding. The semiconductor chips may be bonded using either flip-chip bonding or wire bonding. The resulting packages are referred to as ball grid array (BGA) modules. A plurality of chips having different functions may be integrated in a same BGA module to form a system-in-package (SIP) module.
The BGA modules may be mounted onto printed circuit boards (PCBs) through BGA balls, which are attached to the bond pads on the surface of the BGA modules. In a typical surface mounting process, a BGA module is first placed onto a PCB. A re-flow is then performed so that the BGA balls are melted to join the BGA module to the PCB.
The conventional surface mounting technology suffers from drawbacks. Referring to FIG. 1, BGA module 10, which includes BGA balls 12, is placed on PCB 14. During the re-flow process, the temperatures of BGA module 10 and PCB 14 are elevated. As a result, warpage occurs to BGA module 10. With the warpage, although BGA balls 12 at the center of PCB 14 may still contact PCB 14, the edges of BGA module 10 curve up, and hence are spaced apart from PCB 14. The BGA balls 12 at the edges of BGA module 10 thus may not be able to contact PCB 14. The situation is further worsened by the warpage of PCB 14. During the re-flow, the edges (or corners) of PCB 14 are clamped to the underlying platform (not shown), and hence due to the thermal expansion, PCB 14 warps with the edges curving down. This further increases the distance between the edges of BGA module 10 and PCB 14.
FIG. 2 illustrates a cross-sectional view of the structure shown in FIG. 1, wherein the cross-sectional view is obtained from a vertical plane crossing line 2-2 in FIG. 1. For illustration purposes, the warpage of BGA module 10 and PCB 14 are exaggerated. It is noted that due to the warpage, BGA balls 12 (refer to BGA balls 121), which are located away from the center of BGA module 10, become elongated. This is often characterized as insufficient solder although the real problem is not caused by the shortage of solder. Rather, the problem is that at the locations of BGA balls 121, the spacing between BGA module 10 and PCB 14 is greater than designed. In addition, at the edges of BGA module 10, BGA balls 122 cannot even contact the pads on PCB 14, and hence an open circuit occurs. A solution is thus needed to solve the above-discussed problems to increase production yield.